As for API documentation, I'm hoping to see more of Ember-Data API in the future. The guide as is, is great and you guys done it very well, but some points which were already raised elsewhere (as in "I don't understand Ember" blog posts/rants out there) like dependencies and all things to get started should be addressed as soon as possible. I would like to say the topics that address "getting started" scenarios should be very, very, very detailed, to the point of explaining things that most people would consider obvious.
You can call me n00b and that's fine, but I really like Code School's approach on interactive tutorials; something along those lines would be really good.
Just to put in perpective, the ASP.NET website has a community area with articles and blog posts from the ASP.NET team and from community. Perhaps Ember could have a sort of "community spotlight" as well, or even better: Integrate the Ember Watch into Ember website and let people contribute with articles.
Another thing that I think would be helpful: allow comments/contributions to the guides and API (will require moderation). And perhaps one of those "was this topic helpful?" type of thing too.
Another things that sometimes bites people is the wrong notion that Ember is "for Rails" only when it's totally not the case. It should be clear (I know it is, but... ) that Ember is backend agnostic, and perhaps it should be "louder" in the guides/docs/website. Perhaps talking to influent people on different development communities to help creating starter kits aiming "XYZ" backend would be good too.
As for volunteering, I'd love to be more involved with the community and help in anyway I can. I was thinking of translating docs to Brazilian Portuguese, but I'm not sure how much helpful this would be at this point (not that it's super important since the Internet speaks English anyway). I would also like to reproduce the exact same sample apps you guys are baking but in a different backend, like .NET which is my background. I'm up for whatever if that means helping.